The invention relates generally to control systems for controlling the cyclical motion of fluidically driven members. More specifically, the invention relates to a control system for controlling straight-line shear for cutting gobs from a stream of molten glass in a glassware forming machine.
The use of shears for cutting gobs from a stream of molten glass falling from a feeder of a glassware forming machine is well-known in the prior art. Such shears are generally of either the arcuate or straight-line type, each type having a cyclically oscillating single pair of opposed blades associated with a single stream of molten glass. An example of a prior art double gob shear assembly of the straight-line type is shown in U.S. Pat. No. 4,174,647, dated Nov. 20, 1979, and assigned to the assignee of the present invention. Prior art shears are generally activated by pneumatic (or fluidic) means interconnected in a complex arrangement of various valves and cylinders.
The present invention relates to an improvement over the prior art control system of commonly assigned U.S. Pat. No. 4,467,431, the disclosure of which is incorporated by reference herein. Such patent discloses a control system for pneumatically driven straight-line shears (such as shears of the type disclosed in the U.S. Pat. No. 4,174,647 patent cited above), such control system incorporating a series of sensors which produce signals representing predetermined shear positions in each shear cycle. As illustrated in FIGS. 5 and 6 at the U.S. Pat. No. 4,467,431 and the text discussing these Figures, such prior art system incorporates four sensors--a return delay sensor, a zero crossover sensor, an overlap sensor, and a cushion sensor, each of which produces an output signal at a predetermined point of the travel of the shears, for controlling various aspects of the shears' operation in accordance with the method disclosed therein. The shears are pneumatically driven in the forward (cutting) and reverse direction by selectively energizing and deenergizing (or reversing) the cylinder assembly of FIG. 3. The timing of this energization and deenergization, and hence the motion profile of the shears, is determined by the timing of the return sensor and overlap sensor signals, as well as by a return delay time which is added to the return sensor signal to determine the off time of the shears' pneumatic drive.
As illustrated in FIGS. 6 and 7 of the U.S. Pat. No. 4,467,431 and discussed at column 6, lines 17 and following, the return delay time is calculated by a circuit which incorporates an up/down counter, the count from which decrements or increments as a function of the overlap signal from the previous cycle. When an overlap sensor signal occurs (signifying that the shears have travelled at least as far as such sensor), a "count down" command is produced causing the return delay time to be decremented; the converse happens if such sensor is not triggered in the previous cycle. Although this system provided excellent performance as compared with prior art shears control systems, the continual incrementing and decrementing of the return delay time has imposed a limitation on the accuracy with which shears travel could be controlled.
As an improvement to the assignee's commercial system based upon the invention of the U.S. Pat. No. 4,467,431, an additional, "excess overlap" sensor was included in the sensor housing in order to detect the travel of the shears blades beyond the desired point of maximum overlap to a predetermined point at which an alarm is triggered warning the operator that a reset of the drive electronics or other corrective action is required. Other improvements, not directly relevant to the present invention, are the inclusion of a further sensor which detects the return of the shears to a given, small interval from the base-line or equilibrium position, and the incorporation of a "cushion delay" circuit which acts in cooperation with such further ("park") sensor to enable the operator to adjust the cushioning of the shears return stroke. This drastically reduced the possibility of excessive impact in the system illustrated in the U.S. Pat. No. 4,467,431.
Accordingly, it is a principal object of the invention to provide an improved system for controlling the operation of pneumatically driven straight-line shears. In particular, it is desired to improve the accuracy with which the maximum travel of the shears can be controlled.
Another object is to reduce the normal variation of shears travel, thereby providing more reliable gob drop characteristics and other performance advantages.